1) Field of the Invention
This invention relates to a gel-like emulsion comprising a surface active agent phase as a continuous phase. More particularly, the invention relates to a gel-like emulsion which can readily yield O/W emulsion of high quality by addition of an aqueous phase and to the thus obtained O/W emulsion.
2) Description of the Prior Art
Emulsification methods for obtaining a fine, uniform emulsion generally include a phase inversion emulsification method using nonionic or ionic surface active agents and a gel emulsification method using nonionic surface active agents.
According to the phase inversion emulsification method, a surface active agent is dissolved or dispersed in oil, followed by addition of water phase to the system. As water phase is added, the system is inversed from a W/O state to an O/W state. Through this phase inversion, a fine and uniform emulsion is obtained.
However, there is a limit to an amount of the oil which can be emulsified by a certain amount of surface active agent, and relatively a large amount of surface active agent is required to obtain an emulsion consisting of fine particles. Further, in order to obtain an emulsion in a proper state, it is necessary to adjust a hydrophilic lipophobic balance (HLB) of the surfactant system depending upon a type of oil to be emulsified. In addition, since a particle size of emulsion depends largely on mechanical shear which is imposed to the system during the emulsification, there are some cumbersome problems for determining conditions for the emulsion preparation.
According to the gel emulsification method using nonionic surface active agents, O/W emulsions are obtained by dissolving a nonionic surface active agent in a water-soluble solvent with or without addition of an aqueous phase to give a continuous surface active agent phase, gradually adding an oil to the continuous phase to obtain a gel-like emulsion, and further adding water to the gel-like emulsion. A fine and uniform emulsion can be obtained by this method, however it is necessary to select a proper nonionic surface active agent suitable for the oil to be emulsified. Since H/L balance of a nonionic surface active agent is much affected by temperature, the state of emulsion also depends upon the emulsification temperature. Therefore, in preparing and preserving the above described gel-like emulsion, as well as in producing an O/W emulsion by using the gel-like emulsion and preserving it, temperature is one of very important factors with regard to the emulsion stability. For instance, there is a case where even if a gel-like emulsion can be formed at a low temperature, it cannot be formed at a higher temperature, or an O/W emulsion prepared by a gel-like emulsion is unstable at a high temperature. The gel emulsification method using nonionic surface active agent thus involves such disadvantages on production and quality of emulsion.